1. Field of the Invention
The present invention relates to a reflection type liquid crystal display device and a display apparatus therewith, in particular, to a reflection type liquid crystal display device and a display apparatus therewith having a high contrast ratio and a high display quality.
2. Description of the Related Art
In recent years, the number of pixels of liquid crystal display devices has been increased and in particular the size thereof have been reduced. For example, a fabrication of a liquid crystal display device with a 0.7 inch panel on which as many as 300,000 pixels are formed has been studied. To do that, in such a liquid crystal display device, thin film transistors (TFTS) composed of polysilicon should be formed on a peripheral portion of a substrate with display pixel electrodes and so forth. In addition, the aperture ratio of the pixel portion should be increased.
When the pixel size of the liquid crystal display apparatus is reduced, the decrease of aperture ratio should be considered as an important problem.
In other words, in a conventional liquid crystal display device, light is entered from the opposite substrate. A liquid crystal layer is used as a light shutter. Light passes through the light shutter and is emitted from the array substrate (namely, the front surface of the panel). In this case, the aperture ratio of each pixel is at most 30 to 40%. The rest (60 to 70%) of the pixel is a light shielding layer that shields incident light.
If a switching device that turns on and off a pixel electrode is constructed of a TFT (thin film transistor) composed of for example amorphous silicon, when light strikes the TFT, leak current takes place due to an optical excitation effect. Thus, the operation characteristics of the switching device decrease. Alternatively, the switching device may malfunction. To prevent this problem, a light shielding film that shields the switching device from the light is required. When the light shielding region includes portions such as signal lines and scanning lines composed of metal materials, the region accounts for as many as 60 to 70% of the effective display area.
Thus, as the pixel size decreases, the luminance of the display screen decreases and the contrast ratio degrades. Particularly, in a projection type liquid crystal display device, the contrast ratio of more than 100:1 is required. Thus, when the display area of the pixel becames small, a more critical problem takes place. Consequently, for the liquid crystal display device with small pixels, the efficiency of the use of the incident light should be further improved.
As a means for solving this problem, the advantages of so-called reflection type display device that reflects the incident light on the pixel electrode and emits the light to the incident side are becoming attractive. The reflection type display device has been studied and developed as the liquid crystal display device that should have a higher pixel aperture ratio.
In the reflection type liquid crystal display device, various structural portions including switching devices, scanning lines, and signal lines are disposed below pixel electrodes. Light reflected on the pixel electrodes is displayed on a screen. Thus, the aperture ratio of the pixels of the reflection type liquid crystal display device is theoretically 100% except dividing region of pixels. Although the aperture ratio depends on the panel size, so far it has been improved up to 85%.
However, in the reflection type liquid crystal display device, light entered from the image display side is reflected on the pixel electrodes and displayed on the screen. Thus, in addition to the light that is reflected on the pixel electrodes, undesired light takes place on the front surface of a base member of the liquid crystal display panel. The undesired reflected light dazzles the display screen, thereby remarkably degrading the on/off ratio of the displayed image
The theoretical reflectivity on a transparent base member of the opposite substrate is given by the following equation. EQU r.sup.2 ={(n1-n2)/(n1+n2)}.sup.2
where r.sup.2 is the reflectivity on the base member; and n1 and n2 are the refractive indexes of a first medium and a second medium, respectively.
When light enters from air (refractive index n1=1.0) to a transparent substrate such as a glass substrate (refractive index n2=1.5), the reflectivity is about 4%.
Thus, even if the reflection efficiency of the reflection pixel electrodes is improved to 100% and the absorption loss of light in the liquid crystal layer and the optical path is zero, the on/off contrast ratio of the displayed image that is finally observed is 96%:4%=24:1. Consequently, the contrast is insufficient for both the projection type liquid crystal display device that requires an on/off contrast ratio of more than 100:1 and the direct view type liquid crystal display device.
As a means for preventing light from reflecting on the interface in between different materials in the display device, a reflection protecting film may be coated on the liquid crystal display panel. However, the effect of such a reflection protecting film is limited to a region of a particular wavelength. In addition, since the fabrication of the liquid crystal display panel becomes complicated, the cost thereof increases.
In addition, even if such a reflection protecting film is used, as shown in a curve of contrast ratio v.s. aperture ratio of FIG. 18, undesired reflected light cannot be suppressed enough. Thus, the above-mentioned high contrast ratio of 100:1 cannot be actually accomplished.
For example, in a three-panel type liquid crystal projector, liquid crystal display panels are directly contacted with a dichroic prism. Alternatively, the panels are adhered to the dichroic prism with an adhesive region with almost the same refractive index as that of a glass substrate. In such a manner, the amount of reflected light can be halved.
However, a incident light in the dichroic prism is reflected on the front surface of the dichroic prism. The reflected light is emitted from the dichroic prism along with signal light reflected from the panels. The resultant light is projected to the screen through a projection lens system. Thus, the contrast ratio remarkably deteriorates due to the undesired reflected light, thereby adversely dazzling the screen.
The present invention is made to solve such problems. An object of the present invention is to provide a reflection type liquid crystal projector having a reflection type liquid crystal display device that separates light for image display from light that is simply reflected on the surface of crystal display panel and prevents a contrast ratio from deteriorating due to reflected light so as to display an image with a high contrast ratio.